Embryos in Deep Time

The Rock Record of Biological Development

Marcelo Sánchez-Villagra

Publication Year: 2012

How can we bring together the study of genes, embryos and fossils? Embryos in Deep Time is a critical synthesis of the study of individual development in fossils. It brings together an up-to-date review of concepts from comparative anatomy, ecology and developmental genetics, and examples of different kinds of animals from diverse geological epochs and geographic areas.

Can fossil embryos demonstrate evolutionary changes in reproductive modes? How have changes in ocean chemistry in the past affected the development of marine organisms? What can the microstructure of fossil bone and teeth reveal about maturation time, longevity and changes in growth phases? This book addresses these and other issues and documents with numerous examples and illustrations how fossils provide evidence not only of adult anatomy but also of the life history of individuals at different growth stages. The central topic of Biology today—the transformations occurring during the life of an organism and the mechanisms behind them—is addressed in an integrative manner for extinct animals.

Cover

Title Page, Copyright

Contents

Acknowledgments

I would like to thank the University of Zürich and its Faculty of
Science, as well as the Institute of Paleontology, for providing
an inspiring, challenging, and supportive environment in which
I could write this book. ...

Prologue

The diversity of life is usually presented in evolutionary trees:
A branching pattern culminates in figures of animals and plants.
This is good, as trees convey the common history that organisms,
including ourselves, share. But there is a limitation in this
kind of representation. ...

1. Fossils, Ontogeny, and Phylogeny

I remember as a child being very impressed by a statement,
attributed erroneously to Thomas Huxley, that claimed that if
monkeys were left alone in front of typewriters, they would type
by chance and, given enough time, would indeed type the entire
Encyclopaedia Britannica. I had an abridged version of the Encyclopaedia
in Spanish, fifteen thick volumes, ...

2. Evo-Devo, Plasticity, and Modules

The discipline that brings together the fields of developmental
genetics and evolution has been baptized “evo-devo.” Few if
any new aspects of evolutionary biology have received as much
attention from practitioners and philosophers of biology. Evo-devo
is purported to provide a new kind of synthesis of knowledge
to understand the origin of biodiversity. ...

3. Fossilized Vertebrate Ontogenies

Most fossil remains of vertebrates are mineralized portions of
the skeleton. As the skeleton is at most only partially formed
in embryos and in other juvenile stages, it is not surprising that
most fossils are of adults or subadults, which are also larger than
other life stages and thus more likely to be found. ...

4. Bones and Teeth under the Microscope

As unlikely as it may seem, the most important piece of equipment
for most paleontologists, besides the hammer, is the microscope.
A large proportion of people studying extinct biodiversity
work for the oil industry, examining the very small pollen
of fossil plants or extinct foraminifera, the latter members of
a group of single-celled organisms important for stratigraphic
correlation between geologic sections. ...

5. Proportions, Growth, and Taxonomy

At the time when little anatomical research of microscopic structures
had been done, many people thought that eggs contained
fully formed, very small individuals — or homunculi — a theory
known as preformism (figure 23). Analogous “animalcules”
were assumed for other species. ...

6. Growth and Diversification Patterns

Fossils potentially provide direct evidence on how changes in
growth strategies may have affected diversification patterns
in geologic time. New strategies may have allowed some species
to exploit new ecological opportunities or contributed to
their demise. ...

7. Fossils and Developmental Genetics

Ontogenies do not fossilize. But structures that do were once
the result of a developmental process. Fossils of adult individuals
can then be informative about development by virtue of preserving
phenotypes with an immediate, clear correlation to a
specific developmental process. ...

8. “Missing Links” and the Evolution of Development

Many people are accustomed to thinking of the evolution of life
in terms of a ladderlike progression, with a different animal on
each rung. In the case of vertebrate evolution, they may envisage
a fish on the bottom rung, a salamander on the next, then a lizard,
a mouse, and finally a human on top. ...

9. Mammalian and Human Development

There are about 5,300 species of extant mammals. They represent
only a fraction of the number of species that ever existed
since the separation of the evolutionary line leading to them, at
least 315 million years ago. Then the reptilian and the mammalian
lineages split. ...

10. On Trilobites, Shells, and Bugs

So far I have mostly considered the evolution of vertebrate animals.
There are many more living species of groups of animals
other than vertebrates, and surely the same is true for extinct
species. I aim in this chapter to present some of the discoveries
in this area and the great potential the study of these animals
has. ...

Epilogue: Is There a Moral to Developmental Paleontology?

The history of life is a history of change, and much of that is
recorded by fossils in deep time, in which a vast diversity of
organisms originated and waned. Is there a moral message to be
drawn from this? Of course not. There is no moral intrinsic to a
scientific fact or hypothesis. ...

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